Improvements in jet engine technology over the last half-century have steadily led to a 55% increase in engine fuel efficiency per seat-kilometer since the introduction in the late 1950s of the Boeing 707 aircraft. With the advent of the high bypass turbofan engine, now ubiquitous in civil jet aviation, the fuel consumption per seat-kilometer is about equal to that of early commercial piston-prop airliner engines (data from National Aerospace Laboratory (The Netherlands) report NLR-CR-2005-669). Nevertheless, the low thermal efficiency of these engines (30%-40%) is not sufficient to compensate the rising cost of jet fuel. Low fuel efficiencies require commercial airliners to carry large amounts of fuel on board. A metric that can be used for comparison of overall fuel efficiency is the fuel to payload weight ratio. For example, a Boeing 747-400F requires 266,000 lbs of fuel to carry a payload of 250,000 lbs over a distance of 4,445 nautical miles.
The innovations in jet engine technology that have lead to improvements in fuel efficiency have been historically spurred by spurts in fuel prices, which have negative effects on demand and growth of the industry as airfares are forced to sharply increase. As oil prices had remained steady during the 1980s and 1990s while demand for air travel rose, there was little incentive to make improvements in engine efficiency. Since then, only incremental improvements in engine efficiency have been realized, only coming along after years of pain already experienced by the industry. During the past decade, fuel prices have risen sharply and the trend continues steadily upward. Fuel prices currently account for over half the operating expenses of any airline. The Boeing 787 dreamliner and the Airbus A350 are the industry's most recent answers to rising fuel prices only to be introduced recently, claiming respectively 15% and 25% incremental improvements in fuel efficiency, mostly due to the use of light weight carbon fiber composite structural materials to build the airframe as opposed to fundamental improvements in engine fuel efficiency. Despite these efforts, continually rising oil prices in a volatile and disappearing market will soon outrun any economic gains realized today, demanding again new technologies that will lag the pain in the airline industry as permanent solutions to the petroleum-based fuel crisis are not being proactively sought out.
As a viable alternative to hydrocarbon fuels, hydrogen as a fuel for aviation and ground transport fundamentally has several advantages. It is not subject to the volatility of petroleum pricing, can be produced from green sources and is clean burning, having water vapor as the primary product of combustion. It also has a higher specific heating value than hydrocarbon fuels. For instance, its heating value is 2.75 times higher than that of JP-4 jet fuel. The combustion characteristics of hydrogen are superior to those of hydrocarbon fuels, and therefore internal combustion engines can run significantly more efficiently on hydrogen fuel. Current estimates show that despite complexities in producing and transporting liquid hydrogen, the cost per equivalent gallon of gasoline as low as 1-3 USD, competitive with current jet fuel costs. However, to be carried on board an aircraft, hydrogen must be in the liquid state.
One major impediment to widespread adaptation of liquid hydrogen as fuel for aircraft is its high specific volume, which is approximately 10 times greater than that of hydrocarbon fuels. Therefore a great deal of tankage is required to carry sufficient liquid hydrogen fuel on board for long endurance flight, which would necessitate substantially larger airframes than are used in contemporary airliners. Moving commercial flight airspace to high altitudes (>60,000 ft, or lower limits of the stratosphere) would require airframe designs that provide large wing surface and fuselage size in order to provide sufficient lift and aircraft efficiency to fly at these altitudes. Therefore, the wing and fuselage volume of high-altitude airframes can be significantly larger than those of lower-altitude aircraft, providing potentially adequate fuel storage capacity in the wings and fuselage to carry liquid hydrogen as fuel.
Flying a subsonic aircraft at very high-altitudes nominally requires less energy due to lower drag generated in the thin atmosphere. This can lead to potential gains in fuel efficiency. At altitudes in the stratosphere above 60,000 feet (over 18 km), the atmosphere is very thin (pressures <0.08 bar, or <1 psi, air density <0.1 kg/m3) compared to typical cruising altitudes for conventional airliners at 11 km, where the pressure is ˜0.2 bar (˜3 psi) and air density about 0.35 kg/m3. Aerodynamic craft designed for stratospheric flight encounter little air resistance and dynamic loads, allowing for very large lift-to-drag ratio as well as light-weight fuselage and wing designs to be implemented, leading to increased fuel efficiency.
With the exception of earlier turbojet military aircraft (i.e., Lockheed U2 and Ryan Compass Arrow) designed for subsonic stratospheric flight above 60,000 ft, flight at very high altitudes has been dominated by supersonic turbojet military aircraft. Tubojet engines, while capable of large power densities relative to reciprocating piston engines at low altitudes, have lower subsonic performance at high altitudes in comparison to piston engines driving propellers primarily due to the lower thrust produced as a result of the lack of sufficiently pressurized air. The delivered thrust of these engines at high altitude is on par with that of piston engines operating at the same altitudes. However the thrust-specific fuel consumption is significantly lower for piston engines compared to turbojets. Research and development of highly efficient turbojet engines for high altitude subsonic flight is prohibitively costly, and at the present time the market demand for such aircraft is low. Therefore, to pursue high altitude commercial flight, it is more cost effective to develop reciprocating piston engines for this purpose.
However, conventional spark-ignition reciprocating engines designed for propeller aircraft can function at such high altitudes only with significant modifications, such as the use of turbochargers. Historical examples of this type of engine modification for stratospheric flight are the ALTUS II unmanned aerial vehicle (UAV), developed by General Atomics for NASA, the Boeing-Teledyne Condor and the Grob Aircraft Strato 2C, all having made sustained flights at and above 60,000 ft during the 1990's. These aircraft were equipped with multi-cylinder gasoline (Otto cycle) reciprocating engines (i.e., Drake, Teledyne Continental and ROTAX) and two- or three-stage turbochargers. These modifications generally add significant weight (estimation of almost 3× increase of weight of engine equipped to fly at 85,000 ft vs. at 12,000 ft for same power output) and complexity and size to the engine (number of turbochargers, ducting, cooling heat exchanger stages), and cut operation efficiencies. Summarizing, while both subsonic and supersonic military jet aircraft are more commonly known to fly at very high altitudes, subsonic propeller driven aircraft equipped with conventional piston engine power plants offer greater fuel efficiencies than turbojet engines but with similar power densities at high altitude (Bents et al., “Propulsion System for Very High Altitude Subsonic Unmanned Aircraft” NASA/TM-1998-20636).
Exemplary endeavors have been made to develop high-efficiency propeller driven hydrogen-fueled aircraft for long duration flights at high altitudes (high altitude long endurance, or “HALE” unmanned aerial vehicles, or “UAV”). Examples of these are Aerovironment's Global Observer and Boeing's Phantom Eye and Phantom Ray UAV drones. U.S. Pat. Nos. 6,550,717, 7,281,681, 8,011,616 and 8,028,951 to MacCready et al. (Aerovironment Inc.) all describe a liquid hydrogen-fueled long endurance stratospheric aircraft equipped with high efficiency fuel cells for direct electricity generation to power electric engines driving propellers. Fuel efficiencies are gained from the direct conversion of hydrogen to electricity and many improvements in wing design and fuel storage and delivery. The aircraft is designed to carry a maximum payload of only a little over 100 kilograms. A variation of such an aircraft is described in U.S. Pat. No. 7,806,365 to Miller et al. (Boeing) wherein a liquid hydrogen powered aircraft designed for long endurance stratospheric flight at altitudes over 60,000 ft and carrying up to 700 kilogram payload is equipped with a turbocharged automobile or aircraft piston engine adapted to burn hydrogen. Much of the described invention is concerned with compressor cooling and configuration, as well as fuel handling. While capable of staying aloft for several days at a time, these aircraft are very light-weight and have been designed to carry only small payloads at low airspeeds, mainly to provide high altitude surveillance, communications links and other military or civilian satellite functions at a fraction of the cost of designing, launching and maintaining earth-orbiting satellites.
The above cited examples demonstrate the feasibility and advantages of using liquid hydrogen as a fuel source, despite its low energy density compared to hydrocarbon fuels such as gasoline (9.3 MJ/I compared to 33.5 MJ/I), for HALE flight. While the applications are more strategic and aimed at replacing more costly satellite-based surveillance and communication systems, the economics of carrying such low payloads with such a fuel capacity is not attractive for more common air transport. If the size of the HALE airframes can be modified and scaled up to dimensions more amenable to carry commercial payloads where the aircraft incorporates a highly efficient high power density reciprocating piston engine, the economics for HALE transport planes can become attractive.
The key to opening this doorway lies in the development of such a high efficiency engine. The standard design paradigms for Otto and Miller cycle reciprocating piston engines limit the maximum efficiency that can potentially be obtained. Therefore, the current engine design paradigms must be examined so that new ones can be introduced. To make such engines possible, a new application of engine dynamics and thermodynamics is needed.